System and method for securing a rotor to a motor drive shaft using cam fasteners

ABSTRACT

A rotor assembly for mounting a rotor to a motor shaft having a rotor with a centrally positioned central bore, the diameter of which is slightly larger than a diameter of the motor shaft, a first stepped bore intersecting the central bore and being aligned along a first axis of the rotor, and a second stepped bore also intersecting the central bore and being aligned along a second axis of the rotor. A first cam fastener is disposed within the first stepped bore, and a second cam fastener is disposed within the second stepped bore. Upon rotating the first and second cam fasteners, they engage the motor shaft thereby securely clamping the rotor to the motor shaft.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to rotors, and in particular, to amagnetic encoded rotor incorporating a means for securing the rotor to amotor drive shaft, the means incorporating one or more cam-typefasteners.

2. Related Art

There are various systems and methods for mounting modular magneticencoder rotors to a motor drive shaft (or motor shaft). However, thereare disadvantages with each of these current systems.

A first system is directed to a rotor having an external hub encirclingand extending away from the central aperture. The external hub typicallyhas a diameter less than the outer diameter of the rotor and extendsfrom the face of the hub a relatively short distance. A plurality of setscrews is inserted through holes in the hub to engage the motor shaft.Upon tightening, the set screws secure the rotor to the motor shaft. Inthis embodiment, the external hub is typically positioned external to astator housing, allowing the rotor to be repositioned after the statorhousing is fixed in place. However, a known advantage of a modularencoder is having a thin axial profile, which is defeated by having anexternal hub. An external hub on a rotor adds thickness to the overallrotor, thereby negating the principal advantage of the modular encoder.

The second method is similar to the first system wherein set screws areused to secure a rotor to the motor shaft. In this embodiment, the setscrews are positioned within the working thickness of the rotor suchthat they engage and tighten against the motor shaft through holesextending from the side edge of the rotor through the rotor's entirethickness. This is possible because of the two tracks normally on therotor, one of the tracks is typically a “marker” track which utilizes asmall portion of the rotor's circumference so that the rest of thecircumference on that side is available to locate the set screws. Thisallows the rotor to maintain its thin axial profile. However, there areseveral disadvantages with this rotor design. First, because themagnetic encoder rotor is located inside the stator housing, the sideedge of the rotor is inaccessible after mounting the stator housing. Arotor cannot be repositioned, repaired, or replaced without removing thestator housing. Second, it is more difficult to reliably achieve maximumclamping force with set screw tightening tools.

The third method is a rotor that is mostly hollow, having an internalhub with integral spokes connecting the internal hub with the outer edgeof the rotor. Typically two or three spokes connect the outer diameterto the inner diameter wherein the inner diameter matches the diameter ofthe motor shaft. One of the spokes has a slot in the center, whichextends down through the inner diameter hub. This slot provides theextra space needed to fit the rotor on the motor shaft. A screw passesthrough the spoke with the slot such that upon tightening, the screwpinches the two halves of the slotted spoke together. This pinchingaction constricts the internal hub, tightening the rotor to the motorshaft. This system also has disadvantages. For example, it is moreexpensive to manufacture than using simple set screws. Further, theclamping forces are inferior to those of set screw methods. Lastly, thesize of motor shaft diameters that can be accommodated are limited bythe fact that the space between the slotted spoke and an adjacent spokemust be wide enough for a wrench or other tightening tool to fit.

A fourth system and method for mounting a rotor to a motor shaft is arotor having a tapered hub and is divided into two halves. This systemthen uses a split ring to tighten the hub around a motor shaft. A first,or female, half of the rotor has an inner diameter matching the motorshaft with one side having a tapered groove to the hub defining theinner diameter. A second, or male, half of the rotor is machined with agroove having a taper inverse of the first half. The split ring iscontained within the tapered groove of this second half. Then, as thetwo halves are bolted together with screws, the split ring containedwithin the halves constricts, thereby tightening the hub of the rotoraround the motor shaft. As with other designs, this system for mountingrotor on a motor shaft is more expensive than a set screw system. Also,in the best case, this system has a clamping force comparable to setscrew systems. However, this system is subject to assembly errors. Ifthe clamping screws are not tightened uniformly (or if the screw overthe split in the split ring) is tightened first, the clamping force isseverely limited. There may be no immediate external indication ofimproper clamping, but such an improperly clamped rotor will slip duringnormal operation and cause system failures.

Therefore, there is a need for a simplistic system and method forsecurely mounting a rotor on a motor shaft. There is a further need forsuch a mounting system to provide easy and quick access to the fasteningscrews, or other fastening means, thereby minimizing maintenance time.There is still a further need for a mounting system for a rotor in whichthe rotor retains a thin profile while maintaining maximum clampingforce on the motor shaft.

SUMMARY OF THE INVENTION

The present invention solves the problems associated with traditionalmounting techniques by using cam fasteners, such as cam screws, to mounta rotor to a motor shaft. The principal advantage to using cam screws isthat they provide clamping forces equal to those found in traditionalset screw methods while providing immediate access to the screws formaintenance purposes or for repositioning the rotor on the motor shaftwithout removing the stator housing.

The cam screw rotor contains three pieces. The first is the rotoritself, which is a thin circular disk. The rotor has a central bore witha diameter approximately equal to the diameter of the motor shaft. Twoholes are bored adjacent to the central bore such that each bore isadjacent to, and adjoining with, the central bore. Preferably each boreis aligned with a separate rotor axis. A conventional cam screw (havinga head portion offset from a shaft portion) is screwed into each boreleaving the offset head portion clear of the central bore. Aftermounting the rotor on the motor shaft, the cam screws are rotated suchthat the eccentric heads contact the motor shaft with sufficient torqueto prevent the rotor from slipping.

There are several advantages to this method. First, it is very simple.Manufacturing costs remain low in that the rotor itself has a verysimple, straightforward design and conventional cam screws may be used.Second, the rotor can maintain a very narrow profile because the headportion of the cam screws may fit within a counter bore portion of thebores. Third, the cam screws for clamping the rotor to the rotor shaftare always accessible even after the stator housing is mounted. Fourth,for a given outer diameter, larger motor shaft sizes can be accommodatedthan those using the slotted spoke option. Fifth, this mounting systemachieves clamping forces equal to or greater than that of set screwtechniques, greater then those of the spoke style, and more consistentand less prone to failure than the split ring with tapered hub method.

DESCRIPTION OF THE FIGURES

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawings in which the reference numberfirst appears.

FIG. 1 is a planar front view of a rotor of the present invention;

FIG. 2 is a planar top view of the rotor;

FIG. 3 is a planar cross-sectional side view of the rotor;

FIG. 4 is a perspective view of the rotor assembly of the presentinvention; and

FIG. 5 is a perspective view of a cam fastener.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The rotor assembly of the present invention is shown in the accompanyingfigures. FIGS. 1-3 show the preferred embodiment of a rotor 102 having acentral bore 104 positioned about the intersection of a first, vertical,axis A and a second, horizontal, axis B wherein the first axis A isperpendicular to the second axis B. This central bore 104 has a diameterslightly larger than the diameter of the motor shaft. The rotor 102 hasa first stepped bore 106 consisting of a tapped hole 110 through thethickness of the rotor 102 and a counterbored hole 108. The tapped hole110 and counterbored hole 108 are to accommodate the cam screw describedbelow. The first stepped bore 106 is positioned adjacent to andadjoining with the central bore 104 aligned with the axis A such thatthe counterbored hole 108 intersects with the central bore 104.

In the preferred embodiment, about one quarter of the circumference ofthe counterbored hole 108 intersects with the central bore 104. Inaddition, the center point of the first stepped bore 106 preferablyaligns with the first axis A, but this is for convenience purpose only.The first stepped bore 106 may be positioned anywhere on the centralbore 104 to achieve the same clamping forces needed to securely mountthe rotor 102 to a motor shaft.

The rotor 102 also has a second stepped bore 112 that is very similar tothe first stepped bore 106. That is, the second stepped bore 112,preferably having a counterbored hole 114 and a tapped hole 116, passesthrough the thickness of the rotor 102 and is adjacent to and adjoiningwith the central bore 104 along a portion of its circumference. However,the center point of the second stepped bore 112 preferably aligns withthe second axis B, but this is for convenience purpose only. The secondstepped bore 112 may be positioned anywhere on the central bore 104 toachieve the same clamping forces needed to securely mount the rotor 102to a motor shaft.

As with conventional rotors, the rotor 102 of the present invention hasa first face 118 and a second face 302. In one embodiment, the rotor 102is made of aluminum being about 3.7 inches in diameter and having acentral bore 104 diameter of about 0.6255 inches in diameter, 0.441inches thick, and a first stepped bore 106 and a second stepped bore 112with a counterbored hole 108, 114 of about 0.375 inches in diameter andabout 0.12 inches deep and a tapped hole 110, 116 of about 0.25 inchesin diameter. In addition, the center point of the first stepped bore 106and the second stepped bore 112 is about 0.458 inches in length from thecenter point of the central bore 104 (or the intersection of axis A andaxis B). The edge of the central bore 104 on both the first face 118 andthe second face 302 may optionally have a small taper to facilitate thepositioning of the rotor 102 on a motor shaft. For example, such a tapermay be 0.03 wide having a 45 degree angle.

The present invention is described in terms of a first stepped bore 106and a second stepped bore 112 for convenience purpose only. It would bereadily apparent to one of ordinary skill in the relevant art to use adifferent number of stepped bores in the manner described herein, alongdifferent axes of the rotor 102, to achieve the same clamping forcesneeded to securely mount the rotor 102 to a motor shaft.

The rotor assembly 400 of the present invention of a rotor 102 mountedon a motor shaft 402 is shown in FIG. 4, whereas the preferred camfasteners used for such mounting means are cam screws 404 shown indetail in FIG. 5. The preferred cam screw 404 used in the presentinvention is a conventional cam screw having a head portion 502 and athreaded shaft portion 504 wherein the head portion 502 is offset fromthe central longitudinal axis of the shaft portion 504. A socket 506 inthe head portion 502 is used for tightening and loosening the cam screw404. The preferred embodiment of cam fasteners are shown as cam screws404 for convenience purpose only. The present invention can use anycomparable cam fastener having an offset head.

Operationally, a cam screw 404 is inserted into each of the steppedbores 106, 112 of the rotor 102 and then backed off until the headportion 502 of each cam screw 404 a,b has passed though the central bore104 one time. The rotor 102 is positioned on the motor shaft 402 at adesired location. Once in position, each cam screw 404 a,b is thenrotated until the exterior edge 508 of each head portion 502 engages andis tightened against the motor shaft 402. The exterior edge 508 of a camscrew 404 contacts the motor shaft 402 due to the offset design of thehead portion 502. Rotation of the cam screws 404 a,b is stopped once thedesired clamping force is achieved.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by the wayof example only, and not limitation. It will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined. Thus, the breadth and scope of the present invention should notbe limited by any of the above-described exemplary embodiments.

1. A rotor assembly for mounting a rotor to a motor shaft, comprising: arotor having a thickness, a central bore having a diameter larger thanthe shaft, a first axis passing through a center point of said centralbore, and a second axis passing through said center point of saidcentral bore wherein said first axis is approximately perpendicular tosaid second axis; a first stepped bore having a center point, said firststepped bore being adjacent to and adjoining with said central bore suchthat said center point of said first stepped bore is aligned with saidfirst axis; a second stepped bore having a center point, said secondstepped bore being adjacent to and adjoining with said central bore suchthat said center point of said second stepped bore is aligned with saidsecond axis; a first cam fastener having a length and adapted to fitwithin said first stepped bore; and a second cam fastener having alength and adapted to fit within said second stepped bore; wherein saidfirst cam fastener and said second cam fastener are rotated to clampagainst the motor shaft.
 2. The rotor assembly according to claim 1,wherein said first stepped bore and said second stepped bore each have atapped hole and a counterbored hole.
 3. The rotor assembly according toclaim 1, wherein said first cam fastener and said second cam fastener isa cam screw.
 4. A rotor assembly for mounting a rotor to a motor shaft,comprising: a rotor having a thickness, a central bore having a diameterlarger than the shaft, a first axis passing through a center point ofsaid central bore, and a second axis passing through said center pointof said central bore; and a cam means for securing said rotor to themotor shaft.
 5. The rotor assembly according to claim 4, wherein saidcam means comprises a stepped bore in said rotor and a cam fastener,said stepped bore having a center point and being adjacent to andadjoining with said central bore, wherein said cam fastener is disposedwithin said stepped bore such that a head portion of said cam fastenerengages and clamps against the motor shaft.
 6. The rotor assemblyaccording to claim 5, wherein said cam means comprises a second steppedbore in said rotor and a second cam fastener, said second stepped borehaving a center point and being adjacent to and adjoining with saidcentral bore, wherein said center point of said stepped bore is alignedwith said first axis and said center point of said second stepped boreis aligned with said second axis, wherein said second cam fastener isdisposed within said second stepped bore such that a head portion ofsaid second cam fastener engages and clamps against the motor shaft. 7.The rotor assembly according to claim 6, wherein said first axis isapproximately perpendicular to said second axis.